Speedy can run as fast as the road runner but for a shorter period of time (according to the internet.)

Dashes highest recorded speed was 190 mph in The Incredibles video game.(Only source I could find.)

Sonic is widely know to run at the speed of sound. which is over 700 mph.

The flash can run at the speed of light which is 671,080,888 mph.

1.Flash
2.Sonic
3.Roadrunner
4.speedy(Depending on how long the track is.)
5.dash
Now if you wanna be picky about "what if sonic went super or hyper?"
Well I couldn't find any official source but from what I could gather......
Supersonic speed in MPH is anywhere between 915mph to 3,840mph - all 7 Chaos Emeralds.
Hypersonic speed in MPH is anywhere between 3,840mph to 7,680mph - all 7 Chaos and all 7 Hyper Emeralds. Credit to Boostitude-forums.sega.com/archive

There can be counter arguments for all characters. But when my greatest research source is wiki I'm going by that. If wiki can't give me a average speed. I looked for the most common answer by other people.